Search for Satellites Near Comet 67P/Churyumov-Gerasimenko Using Rosetta/OSIRIS Images

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A&A 583, A19 (2015) Astronomy DOI: 10.1051/0004-6361/201525979 & c ESO 2015 Astrophysics Rosetta mission results pre-perihelion Special feature

Search for satellites near comet 67P/Churyumov-Gerasimenko using Rosetta/OSIRIS images

I. Bertini1, P. J. Gutiérrez2, L. M. Lara2, F. Marzari3, F. Moreno2, M. Pajola1, F. La Forgia3, H. Sierks4, C. Barbieri3, P. Lamy5, R. Rodrigo6,7, D. Koschny8, H. Rickman9,10, H. U. Keller11, J. Agarwal4, M. F. A’Hearn12, M. A. Barucci13, J.-L. Bertaux14, G. Cremonese15, V. Da Deppo16, B. Davidsson9, S. Debei17, M. De Cecco18, F. Ferri1, S. Fornasier13,19, M. Fulle20, L. Giacomini21, O. Groussin22, C. Güttler4, S. F. Hviid23, W.-H. Ip24,25, L. Jorda22, J. Knollenberg23, J. R. Kramm4, E. Kührt23, M. Küppers26, M. Lazzarin3, J. J. Lopez Moreno2, S. Magrin3, M. Massironi21, H. Michalik27, S. Mottola23, G. Naletto28,16,1, N. Oklay4, N. Thomas29, C. Tubiana4, and J.-B. Vincent4 (Aﬃliations can be found after the references)

Received 27 February 2015 / Accepted 11 May 2015

ABSTRACT

Context. The European Space Agency Rosetta mission reached and started escorting its main target, the Jupiter-family comet 67P/Churyumov- Gerasimenko, at the beginning of August 2014. Within the context of solar system small bodies, satellite searches from approaching spacecraft were extensively used in the past to study the nature of the visited bodies and their collisional environment. Aims. During the approaching phase to the comet in July 2014, the OSIRIS instrument onboard Rosetta performed a campaign aimed at detecting objects in the vicinity of the comet nucleus and at measuring these objects’ possible bound orbits. In addition to the scientific purpose, the search also focused on spacecraft security to avoid hazardous material in the comet’s environment. Methods. Images in the red spectral domain were acquired with the OSIRIS Narrow Angle Camera, when the spacecraft was at a distance between 5785 km and 5463 km to the comet, following an observational strategy tailored to maximize the scientific outcome. From the acquired images, sources were extracted and displayed to search for plausible displacements of all sources from image to image. After stars were identified, the remaining sources were thoroughly analyzed. To place constraints on the expected displacements of a potential satellite, we performed Monte Carlo simulations on the apparent motion of potential satellites within the Hill sphere. Results. We found no unambiguous detections of objects larger than ∼6 m within ∼20 km and larger than ∼1 m between ∼20 km and ∼110 km from the nucleus, using images with an exposure time of 0.14 s and 1.36 s, respectively. Our conclusions are consistent with independent works on dust grains in the comet coma and on boulders counting on the nucleus surface. Moreover, our analysis shows that the comet outburst detected at the end of April 2014 was not strong enough to eject large objects and to place them into a stable orbit around the nucleus. Our findings underline that it is highly unlikely that large objects survive for a long time around cometary nuclei. Key words. comets: general – comets: individual: 67P/Churyumov-Gerasimenko – planets and satellites: detection – techniques: photometric

1. Introduction to be composed of objects with an orbit intermediate between TNOs and short-period comets (Levison & Duncan 1997). The detection and study of small-body satellites is an important Spacecraft encounters allow satellite searches and discov- tool for investigating the nature, origin, and evolution of aster- eries down to sizes much smaller than possible from Earth, oids and comets. Measuring the orbit of small companions al- which also adds the advantage of effectively investigating the lows determining the mass of the system and of the primary. space closer to the objects. Several satellite searches were per- From this, its bulk density is derived when the volume is known. formed using data from NASA, JAXA, and ESA missions. We This provides hints on the physical composition of the object mention the studies of NASA/Galileo at (951) Gaspra (Belton and its internal structure. Studying the connected systems also et al. 1992) and (243) Ida (Belton et al. 1995), NASA/NEAR at provides clues on the collisional events that occurred during the (253) Mathilde (Veverka et al. 1999) and (433) Eros (Veverka early stages of the formation of the solar system and its subse- et al. 2000), JAXA/Hayabusa at (25143) Itokawa (Fuse et al. quent evolution (Merline et al. 2002). 2008), ESA/Rosetta at (21) Lutetia (Bertini et al. 2012), and At the time of writing (beginning of May 2015), we know finally NASA/Dawn at (4) Vesta (Memarsadeghi et al. 2013). of 256 small bodies that have companions of different sizes1. Except for the encounter with Ida, which provided the first di- Among them there are 55 near-Earth asteroids (NEAs), 20 Mars- rect and definitive evidence of the existence of asteroid com- crossers, 97 main belt asteroids (MBAs), 4 Jupiter Trojans, and panions with the serendipitous discovery of the small moon 80 trans-Neptunian objects (TNOs). Three TNOs that display Dactyl in 1993, all other searches were unsuccessful in detect- complex systems belong to the Centaur class, which are assumed ing small companions. These studies allowed placing important constraints on the size limit of possible satellites, however, pro- viding hints on the collisional history of the primary bodies. 1 http://www.johnstonsarchive.net/astro/asteroidmoons. A double nucleus with two possibly bound components was html claimed to explain the photometric anisotropies in the inner

Article published by EDP Sciences A19, page 1 of8 A&A 583, A19 (2015) coma of the large comet C/1995 O1 Hale-Bopp based on both Short-exposure frames were taken for their relevance if a large ground-based (Marchis et al. 1999) and HST (Sekanina 1997) satellite had been detected, since they would have provided un- data. However, as underlined in Noll et al.(2006), Weaver & saturated views of the object. The NAC broadband orange filter Lamy(1997) reported no evidence of the second companion us- (with a central wavelength and FWHM of 649.2 nm and 84.5 nm, ing the same HST dataset, showing that no final univocal con- respectively) in the visible red domain was chosen to provide clusion on the binary nature of the system could be derived. the best S/N for possible satellites within a fixed exposure time. Decimeter-sized icy particles were found in the close vicinity Considering the image scale, the observed field of view (FoV) of the nucleus of the hyperactive comet 103P/Hartley2 during covered the inner ∼110 km from the comet optocenter. When the flyby of the NASA/EPOXI spacecraft performed on 2010 we mention a distance, it refers to the “projected distance”. November 4 (A’Hearn et al. 2011; Kelley et al. 2013; Hermalyn Assuming a Hill sphere radius of ∼650 km derived from the et al. 2013). Moreover, several cometary nuclei visited by space measurement of the comet mass by the Rosetta Radio Science missions (e.g., 1P/Halley, 19P/Borrelly, 103P/Hartley 2, and Investigation (RSI) instrument, namely 1.0 × 1013 kg (Sierks 67P/Churyumov-Gerasimenko itself) showed complex irregular et al. 2015, and references therein), and from the heliocentric shapes that can be interpreted as the results of the evolution distance-dependent formula in Hamilton & Burns(1991), we of contact binary systems. Despite these interesting consider- note that our FoV intersected a three-dimensional space corre- ations, no classical satellite searches have been performed for sponding to ∼37% of the comet gravitational sphere of influence. comets, as was extensively done for asteroids, and no solid ma- The first and the last images obtained for the satellite search terial larger than ∼1 m orbiting a comet has ever been unambigu- are shown in Fig.1. ously discovered. During the approach to comet 67P/Churyumov- Gerasimenko in July 2014, before the orbit insertion performed 3. Analyzing the data at the beginning of August 2014, the two-camera instrument OSIRIS (Keller et al. 2007) onboard Rosetta took several When we analyzed the data, we first estimated the expected images with the purpose of detecting and studying possible results for a potential satellite by performing a Monte Carlo objects orbiting the comet in the vicinity of the nucleus. The simulation. To do this, we selected 50 000 clones randomly lo- search had the additional aim to ensure spacecraft safety. The cated inside the Hill sphere, with a random velocity with a discovery of solid blocks close to the nucleus would have smaller modulus than the escape velocity. We calculated the implied that special care was necessary so that the spacecraft clone positions within the CCD sensor reference frame for the trajectory would not cross any orbiting material. An appropriate time in which the long-exposure images were taken. A value observational strategy was defined so that the data acquisition of 1.0 m s−1 was used for the escape velocity, in accordance and reduction processes were optimized, maximizing the pos- with Sierks et al.(2015). In a first approach, acceleration e ffects sibility of detecting and measuring the orbital arc of a possible on the potential satellite were neglected. The spacecraft posi- small companion. We here present the adopted observational tion and frame orientations were derived using appropriate spice strategy and the data analysis, together with the results of our kernels. investigation. After calculating the clone positions, we measured their displacements for three different cases: (1) within a single exposure time to determine whether the potential satellite showed a track 2. Observational strategy in a single frame; (2) between individual images to determine It is well known that satellite searches from spacecraft images whether it was possible to apply median averaging of images are affected by several problems such as the fast motion of the within the same run in order to eliminate possible cosmic rays camera with respect to the target and the bona-fide detection of and spurious signals; (3) between runs of images to limit the ra- interesting point-like objects against background stars, cosmic- dius search for displacements of the potential satellite. ray events, and CCD defects (Merline et al. 2002). With this analysis we found that clones did not move more One OSIRIS image series was specifically devoted to the than 0.5 px within each run (see Fig.2), allowing us to median search for potential satellites during the comet approach phase combine the three frames within each run to effectively eliminate on July 20, 2014, using the high-resolution Narrow Angle cosmic-ray effects. Moreover, the theoretical displacement from Camera (NAC) telescope. The images were taken when the run to run was calculated to be smaller than 50 px within the comet was at 3.69 AU from the Sun. The distance to the comet first five runs, separated by 1 h. The displacement of the poten- decreased from 5785 km to 5463 km from the beginning to the tial satellite between runs 5 and 6 may have been practically any end of the series. The phase angle of the observations was 7◦. value, depending on the clone velocity, because the time separa- The series consisted of 18 short- and 18 long-exposure images tion between these two runs was 7 h (see Fig.3). For this reason, divided into three consecutive frames so as to reduce the contam- we focused on the analysis of the first five runs and kept the sixth ination from cosmic-ray hits through their lack of persistence, run in reserve in case we detected a potential satellite within the for a total of six different short- and long-exposure runs. Each first five runs. To detect potential satellites, we started working run was separated by 1 h except for the last one, which was taken with only the median average of the three long-exposure images 7 h apart. The satellite search series therefore covered almost an of each run. entire comet rotation period. Within each run, the time separa- We then defined the limiting flux for the object detection between consecutive frames corresponded to 20 s except tion. OSIRIS frames are photometrically calibrated using the for the 5th run, where it was 10 s. Within the series, the same frequently tested instrument calibration pipeline described in short- (0.14 s) or long- (1.36 s) exposure time was used to reach Tubiana et al.(2015a). First, we used the SExtractor code (Bertin the same limiting magnitude and avoid difficulties when looking & Arnouts 1996) with a proper set-up for detecting light sources for correspondences among the three frames. The long-exposure with a flux ≥3σ, where σ is the background level, within an times were selected with the aim of avoiding stellar background aperture radius 2 px larger than the filter point spread function smearing, which could have complicated the star identification. (PSF) full width at half maximum (FWHM) that was estimated

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Fig. 1. First and last runs short (a), c)) and long exposure (b), d)) median-combined frames taken for the satellite search on July 20, 2014. The horizontal white bar in a) corresponds to a scale length of 50 km at the comet distance. from calibrations to be ∼1.8 px (Magrin et al. 2015). To en- were identified as the sources located in the same pixel, with a sure full control of the source detections, we performed man- tolerance lower than 5 px. The value of this error depends on ual photometry and estimated the flux in a circular aperture of the geometric distortion correction goodness of the field, on the 2 px radius of all the sources detected by SExtractor. We also central pixel identification error of the sources, and on the corre- estimated the local sky background. We only considered sources lation algorithm itself. with a flux three times the standard deviation of all sky values The remaining sources, found to be ∼200 for each run (see for the subsequent study. This was defined as the source thresh- Table1), might in principle be spurious signals, undetected stars, old. The highest source threshold value of the different runs was CCD defects, and, of course, potential satellites. These remain- set as our detection limit (see Table1). This resulted in consid- ing sources, shown in Fig.4, were considered for further inves- ering light sources with fluxes in the 2 px aperture larger than tigation. The plot shows the sources that were not identified as − − − − 8.9 × 10 8 [W m 2 nm 1 sr 1]. We correlate the fluxes with the stars in the different runs displayed in the same frame, obtained R and V magnitudes of the nucleus below. after correlation. Clouds of sources around the central position The stellar background was then identified by correlating the of the CCD are due to the edge of the window that we were different median-averaged images through small shifts and ro- forced to define to avoid the ghost of the nucleus, which is al- tations to cause the brightest 100 sources found in the images ways present in our images and cannot be considered reliable. with SExtractor to overlap. After correlating the frames, the stars This resulted in cutting out a square of 400 px size centered on

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Fig. 2. Normalized frequency of satellite clone displacements within a single run composed of three consecutive images. Results are shown for all six runs covering the satellite search. Fig. 4. Correlated long-exposure image showing all detected light sources not identiﬁed as stars. A, B, and C display a track but were discarded as potential satellites after the analysis.

were verified to be CCD defects, which also appear in images that are unrelated to the satellite search series and correspond to the same (x, y) position when displayed in the original uncorre- lated frames. The main problem of identifying a satellite close to the detection limit is that the potential satellite is not necessar- ily present in all five considered runs. The satellite can remain undetected in some of the runs, for example, because of intrin- sic rotational variability or high background level. To find and detect possible tracks of potential satellites, we therefore considered the information obtained from the clone simulations. We only took into account for a thorough analysis the sources showing a potential track that appeared in at least three runs out of the five. With the Monte Carlo analysis we verified that all the po- Fig. 3. Normalized frequency of satellite clone displacements between two consecutive runs. Results are shown for displacements between tential satellites appeared to move in a straight line from top to runs 1 and 2 (continuous line), runs 2 and 3 (dotted line), runs 3 and 4 bottom in the CCD frame (see Fig.5) and that the displacement (dashed line), runs 4 and 5 (dashed-dotted line), and runs 5 and 6 from run to run was exactly the same, within at most 2 px. This (dashed-triple-dotted line). The last curve depicts the large displace- “error” would also include the source pixel identification, bear- ment that was the reason for discarding run 6 from the satellite analysis. ing in mind that the PSF FWHM is ∼1.8 px. As the considered space region is beyond 20 km from the nucleus center, gravita- Table 1. Image series dedicated to the satellite search. tional accelerations were not considered in the simulations. We assumed that they may be a second-order effect, on the order of the pixel scale at most. The gravitational acceleration is prob- −2 −1 −1 Image 3σ [W m nm sr ] Sources Non-stars ably lower than 10−5 m s−2 for cometocentric distances larger RUN1 8.3 × 10−8 1316 200 than 6 km from the surface, given the mass of 67P. This accel- RUN2 6.4 × 10−8 1398 221 eration, in modulus, would produce a displacement on the order RUN3 7.2 × 10−8 1260 151 of or smaller than the pixel size scale from run to run (except for RUN4 8.9 × 10−8 1273 185 runs 5 to 6, where the acceleration might have a noticeable effect) and, in any case, smaller than the PSF FWHM. In the search RUN5 8.1 × 10−8 1451 224 for source tracks, we additionally imposed from the clone study Notes. Sources and non-stars are the total number of detected sources that the separation from run to run of the potential satellite had with a flux higher than 3σ and the number of detected sources that are to be of the same order, with a safe margin of 3 px, and that it not identified as stars. could not be larger than 50 px. From our analysis, we found four sources showing a compatible theoretical track, but they were either hot pixels or CCD defects, as confirmed in unrelated im- the comet nucleus, corresponding to the first ∼20 km around the ages (as the tracks identified by A and B in Fig.4). After a thor- comet optocenter and to the inner ∼4% of the Hill sphere. In ough search, the most likely track for a candidate satellite was the displayed frame, a satellite would appear as an apparently the track identified by the letter C in Fig.4. This track consisted moving object showing a track corresponding to its position at of three sources appearing in runs 3, 4, and 5, as indicated in the different runs (e.g., points A and B in Fig.4). These tracks Table2. This potential common source appearing in at least three

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Table 2. Candidate satellite data.

Image Position1 Distance1 [px] Position2 Distance2 [px] Flux [W m−2 nm−1 sr−1] RUN3 (1039, 303) (1075, 214) 2.3 × 10−6 RUN4 (1043, 308) 7 (1091, 177) 41 4.5 × 10−7 RUN5 (1039, 315) 8 (1109, 137) 43 1.7 × 10−6

Notes. Positions 1 and 2 are the CCD (x, y) coordinates of the candidate satellite in the original and correlated frames. Similarly, distance 1 and 2 are the separations of the sources in the different runs in the original and correlated frames. The keyword flux indicates the photometric flux of the object. that we found no unambiguous detections of objects in the ∼[20−110] km range from the nucleus up to a limiting flux of 8.9 × 10−8 [W m−2 nm−1 sr−1].

3.1. Space close to the nucleus To analyze the space close to the nucleus, which was cut off in the long-exposure images because of the bright ghost of the nucleus, we took into account the short-exposure frames belonging to the same series. We applied the same analysis as performed on the long- exposure images, this time considering the entire frame, includ- ing the space close to the nucleus and the nucleus itself. Since we aimed to study the region close to the nucleus, we performed additional simulations with clones that had a Keplerian acceleration according to their distance from the nucleus. Even though acceleration may be comparatively large close to the nucleus, its effect was not noticeable in the images we considered since it was found to be smaller than 1 px in all images given the short exposure time. Based on our simulations, we therefore conclude that the constraints imposed from the long-exposure images still hold. Fig. 5. Apparent motion of 50 satellite clones in the correlated frame The main problem here was defining a proper detection from the first image of run 1 to the last image of run 5. The arrow orien- threshold because the electronic noise resulted in background tation indicates the direction of the apparent motion in the CCD frame patterns that hindered defining a limiting S/N. taking into account the spacecraft position and pointing as included in We therefore relaxed the SExtractor detection limit. With a the spice kernels and a random velocity lower than the escape velocity. trial-and-error procedure, we defined the signal of 4.5σ (σ is al- The arrow length provides an indication of the total displacement on the CCD frame due to the satellite clone motion. ways the background level) as the lowest value that produced reliable detections. This resulted in considering light sources with a flux higher than 3.0 × 10−6 [W m−2 nm−1 sr−1]. runs would have had a separation from run to run that was very This search yielded the correlated image shown in Fig.6. close to the maximum of the expected separation from the clone In that figure, several apparent moving objects can be clearly study (see Fig.3), and the di fference in distance from runs 3–4 identified. Four of them, as those labeled A and B, correspond to runs 4–5 was smaller than 2 pix. The related photometric flux to the aforementioned CCD defects. The object labeled E is the showed a variation of one order of magnitude, which might be comet nucleus, and D is a spurious detection due to the nucleus due to a very elongated shape (it would correspond to an axis ra- ghost. tio of ∼ 3). All these circumstances defined this potential source We therefore finally conclude that we found no unambiguous as a possible candidate. Nevertheless, this track showed a pe- detections of objects within ∼20 km from the nucleus up to a culiarity observable when the (x, y) coordinates in the original limiting flux of 3.0 × 10−6 [W m−2 nm−1 sr−1]. CCD images were considered. The potential source was characterized by a small swaying movement in the (x) coordinate. This movement had a significant amplitude of 4 pixels, larger than the 4. Estimating the limiting size PSF FWHM. The clone study was used to detect possible satellites with such an apparent motion, and we found no single case Since our search for objects in the vicinity of the comet produced characterized by this apparent displacement. Additionally, if the a negative result, we determined the limiting size for any object change from run 3 to run 4 is considered, the expected position at that might be present, but remain undetectable in our images. run 5 should have been (1047, 313) in the original CCD image, that is, more than five times the PSF FWHM in the (x) coordi- 4.1. Measuring the limiting magnitude nate. These arguments led us to conclude that the apparent track was just a coincidence and did not correspond to a real source To determine the limiting size of possible solid blocks for de- detected in three out of the five runs. tection we first measured the limiting magnitude reached in our Based on our thorough analysis of the remaining sources images both within ∼20 km and from ∼20 km up to ∼110 km and imposing the described constraints, we therefore conclude from the comet optocenter.

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Table 3. Final limiting sizes for potential satellites of 67P.

Image texp [s] Size [m] Error [m] RUN1 0.14 5.77 0.23 1.36 1.00 0.11 RUN2 0.14 5.73 0.24 1.36 0.99 0.10 RUN3 0.14 5.69 0.24 1.36 0.98 0.10 RUN4 0.14 5.66 0.23 1.36 0.98 0.11 RUN5 0.14 5.62 0.21 1.36 0.97 0.10

Notes. texp is the image exposure time within single runs. Size and error columns show our ﬁnal results in measuring the limiting size of undetected objects and the associated error estimate.

Our final results are shown in Table3 together with the associated error estimates. The largest contribution to the error on Fig. 6. Correlated short-exposure image showing all detected sources the size measurement comes from considering the variation of that are not identified as stars. A and B display a track but were dis- the observational geometry within the Hill sphere for the vir- carded as potential satellites since they were identified as CCD defects. tual satellites. The propagated error due to the uncertainty on the Objects labeled E and D are the comet nucleus and a spurious detection radiometric measurements coming from the OSIRIS calibration due to the nucleus ghost. pipeline is negligible when compared to the effect of the variation of the observational geometry. For this reason, we report as First, we converted the measured NAC broadband orange fil- final results the mean values of the size distribution found within ter limiting fluxes into Kron-Cousins R magnitudes. This was the Hill sphere for every single run, and as associated error three performed using the OSIRIS standard calibration fields, as in times the stardard deviation of the size distribution. We conclude Mottola et al.(2014). Our limiting fluxes of 8 .9 × 10−8 and that 67P lacks objects larger than ∼6 m within the first ∼20 km −6 −2 −1 −1 from the comet nucleus and objects larger than ∼1 m at cometo- 3.0 × 10 [W m nm sr ] corresponded to Rlim = 14.63 centric distances between ∼20 km and ∼110 km at the time the and Rlim = 10.81, respectively. These are the smallest magnitudes that a satellite would have observations were performed. to have to be detected in the two different regions around the comet. Considering objects with the same photometric proper- 5. Summary and discussion ties as the nucleus of 67P, the limiting magnitude in R can be converted into a V-Johnson magnitude using the Johnson-Kron- Images were taken with the aim to detect objects orbiting comet Cousins colors of the comet nucleus. Based on (V − R) = 0.54 67P/Churyumov-Gerasimenko on July 20, 2014, by the OSIRIS (Tubiana et al. 2011), Rlim = 14.63 and Rlim = 10.81 translated NAC telescope onboard the Rosetta mission during the approach into Vlim = 15.17 and Vlim = 11.35, respectively. to the comet at ∼5800 km distance both for scientific and spacecraft security reasons. The negative outcome of our search led us to estimate a limit- 4.2. From limiting magnitude to limiting size ing size for possible undetected objects with the same photomet- After calculating Vlim, we were able to derive the absolute limit- ric properties as the comet nucleus. We found no unambiguous ing magnitude, Hlim. To do this, we used the photometrical mod- detections of objects larger than ∼6 m at within the first ∼20 km els developed by Bowell et al.(1989). We used the hypothesis of from the nucleus and objects larger than ∼1 m at cometocentric a satellite that has the same photometric properties as the comet distances between ∼20 km and ∼110 km. nucleus and considered the real geometry of observations to con- There are three most likely mechanisms that might produce strain the maximum distance and minimum phase angle of a pos- a satellite for comet 67P: a subcatastrophic impact generating a sible satellite detected by the camera. cloud of fragments that re-accumulate into a satellite before re- The input photometric slope-parameter, G = −0.13 ± 0.01 impacting, comet splitting due to internal stresses caused either was measured from OSIRIS unresolved images of the comet nu- by activity (i.e., 73P/Schwassman-Wachmann 3), or tidal forces cleus taken during the approaching phase (Fornasier et al. 2015). (i.e., comet Shoemaker-Levy 9), and radial gas drag forces lift- To provide the observational geometry input needed to con- ing up boulders. vert Vlim into Hlim, we measured the three-dimensional positions In the first case, the impact would have to have occurred of 100 000 Monte Carlo virtual satellites filling up the entire Hill when the comet was residing in the Kuiper Belt. Low-velocity sphere. The absolute limiting magnitude was then converted into cratering impacts (∼1 km s−1) within the belt may cause the ejec- a diameter measurement using (Chesley et al. 2002): tion of a large number of small fragments into temporary orbits. The comet irregular shape and complex gravitational field may 1329 × 10−0.2H allow these fragments to survive for more than one period before D[km] = √ , (1) pV falling back onto the comet, thus giving them enough time to col- lide with each other and accrete into a satellite. This mechanism where pV = 0.061 ± 0.001 is the geometric V-band albedo of the would not be efficient if the comet originated from the Oort comet nucleus (Fornasier et al. 2015). cloud.

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Cometary splitting is a common event: more than 40 split by many adverse events. Close encounters with Jupiter, like the comets have been observed in the past 150 yr. The peak in the very deep one of 1959, may destabilize a satellite orbit through location of the breakup is close to perihelion at about 2 AU from tidal forces and cause it to depart. In addition, changes in the the Sun (Boehnhardt 2004). The relative speed of the fragments pole direction due to strong outgassing close to perihelion cause shortly after the fragmentation event usually appears to be high significant variations in the gravity field (which is very irregular and not favorable to capture one or more fragments as satellites. for 67P), and the satellite orbit may become unstable, leading to However, it is possible that a whole spectrum of separation ve- escape. Sublimation on a potentially small satellite would also locities is covered during the splitting event, and a particular strongly reduce its endurance, causing its fast erosion. The non- combination of low-separation velocity, complex gravitational gravitational forces related to the sublimation and the gas pres- field, and outgassing force may inject a small component into a sure released from the comet nucleus would also contribute to bound orbit. destabilizing its orbit. Radial gas drag forces may lift meter–sized boulders from In conclusion, even if there are different mechanisms that can the nucleus surface, possibly injecting them into bound orbits, cause the formation of a comet satellite, the adverse dynamical in particular if large anisotropies are present in both the gas conditions that characterize the comet environment seem to play drag and gravity force (Fulle 1997), as seems to be the case of against its survival. comet 67P. Isolated boulders with a size of at most ∼6–7 m that are linked to possible gas activity ejection processes are seen in several areas on the nucleus surface (Pajola et al. 2015). After Acknowledgements. OSIRIS was built by a consortium of the Max-Planck- having been lifted up, several boulders might survive for one Institut für Sonnensystemforschung, Göttingen, Germany, CISAS – University or more full orbital periods of the comet and could appear as of Padova, Italy, the Laboratoire d’Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and small satellites. However, the sensitivity of our images allowed Scientific Support Department of the European Space Agency, Noordwijk, The excluding objects with sizes larger than a few meters in late Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, July 2014. This might be an indication that either the outgassing the Universidad Politéchnica de Madrid, Spain, the Department of Physics and has not been strong enough to lift large boulders before that Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und date or that the orbital survival of these objects through comet Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France perihelion is difficult because of the strong perturbations from (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical continuing outgassing. We also emphasize that our search only Directorate is gratefully acknowledged. covered the inner ∼37% of the comet’s Hill sphere calculated at the time of observations. Nevertheless, the images taken in July 2014 cover ∼87% of the Hill sphere calculated at perihelion (rHill = 215 km), where the efficiency in ejecting objects is References highest. Our conclusions are therefore valid within the full ex- A’Hearn, M. F., Belton, M. J. S., Delamere, W. A., et al. 2011, Science, 332, tension of the comet’s gravity field throughout its entire orbital 1396 period. Belton, M. J. S., Veverka, J., Thomas, P., et al. 1992, Science, 257, 1647 Our results are consistent with the findings in Rotundi et al. Belton, M. J. S., Chapman, C. R., Thomas, P. C., et al. 1995, Nature, 374, 785 Bertini, I., Sabolo, W., Gutierrez, P. J., et al. 2012, Planet. Space Sci., 66, 64 (2015), where a cloud of ∼350 dust grains bound to the comet, at Boehnhardt, H. 2004, Split comets, ed. G. W. Kronk, 301 nucleocentric distances lower than 130 km, was found in NAC Bowell, E., Hapke, B., Domingue, D., et al. 1989, in Asteroids II, eds. R. P. orange filter images taken on August 4, 2014. The authors es- Binzel, T. Gehrels, & M. S. Matthews, 524 timated these dust grains to have probably been placed in orbit Chesley, S. R., Chodas, P. W., Milani, A., Valsecchi, G. B., & Yeomans, D. K. 2002, Icarus, 159, 423 just after the previous perihelion passage and to span from 4 cm Davidsson, B., Gutiérrez, P. J., Sierks, H., et al. 2015, A&A, 583, A16 to ∼2 m in size, being the last a crude upper limit obtained as- Fornasier, S., Hasselmann, P. H., Barucci, M. A., et al. 2015, A&A, 583, A30 suming that the brightest detected grains are also the farthest Fulle, M. 1997, A&A, 325, 1237 from the spacecraft. Taking into account the errors associated Fuse, T., Yoshida, F., Tholen, D., Ishiguro, M., & Saito, J. 2008, Earth, Planets, with the size measurements in these two independent works, our Space, 60, 33 Hamilton, D. P., & Burns, J. A. 1991, Icarus, 92, 118 satellite search analysis confirms that objects larger than the up- Hermalyn, B., Farnham, T. L., Collins, S. M., et al. 2013, Icarus, 222, 625 per limit in Rotundi et al.(2015) were not present in the vicinity Keller, H. U., Barbieri, C., Lamy, P., et al. 2007, Space Sci. Rev., 128, 433 of the nucleus. Similar considerations are valid when comparing Kelley, M. S., Lindler, D. J., Bodewits, D., et al. 2013, Icarus, 222, 634 our findings with the results of Davidsson et al.(2015), where Levison, H. F., & Duncan, M. J. 1997, Icarus, 127, 13 Magrin, S., La Forgia, F., Da Deppo, V., et al. 2015, A&A, 574, A123 the orbits of a few grains around the nucleus, with a size in the Marchis, F., Boehnhardt, H., Hainaut, O. R., & Le Mignant, D. 1999, A&A, 349, [0.14–0.50] m range, were calculated using OSIRIS Wide Angle 985 Camera images taken on September 10, 2014, in the narrow– Memarsadeghi, N., McFadden, L. M., Skillman, D., et al. 2013, Proc. 2012 IS band visible filter. and T/SPIE Electronic Imaging, Computational Imaging X Conference Moreover, OSIRIS detected a clear comet outburst between Merline, W. J., Weidenschilling, S. J., Durda, D. D., et al. 2002, Asteroids III (Tucson: University of Arizona Press), 289 2014 April 27 and 30, 2014, during the approaching phase. This Mottola, S., Lowry, S., Snodgrass, C., et al. 2014, A&A, 569, L2 impulsive event was estimated to have ejected a mass between Noll, K. S., Levison, H. F., Grundy, W. M., & Stephens, D. C. 2006, Icarus, 184, 103 kg and 105 kg (Tubiana et al. 2015b). Our results indicate 611 that the forces produced by the outburst were unable to lift up Pajola, M., Vincent, J.-B., Lee, J.-C., et al. 2015, A&A, 583, A37 Rotundi, A., Sierks, H., Della Corte, V., et al. 2015, Science, 347, 3905 large chunks of material or that such blocks were unable to en- Sekanina, Z. 1997, Earth Moon Planets, 77, 155 ter into orbit and remain close to the nucleus at the time of Sierks, H., Barbieri, C., Lamy, P. L., et al. 2015, Science, 347, 1044 the satellite observations, three months after the impulsive event Tubiana, C., Böhnhardt, H., Agarwal, J., et al. 2011, A&A, 527, A113 occurred. Tubiana, C., Güttler, C., Kovacs, G., et al. 2015a, A&A, 583, A46 Tubiana, C., Snodgrass, C., Bertini, I., et al. 2015b, A&A, 573, A62 Finally, considering all plausible formation scenarios, even Veverka, J., Thomas, P., Harch, A., et al. 1999, Icarus, 140, 3 if a satellite larger than a few meters was formed during the evo- Veverka, J., Robinson, M., Thomas, P., et al. 2000, Science, 289, 2088 lution of the comet, its survival would have been jeopardized Weaver, H. A., & Lamy, P. L. 1997, Earth Moon Planets, 79, 17

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14 LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 1 Guyancourt, France Center of Studies and Activities for Space (CISAS) “G. Colombo”, 15 University of Padova, via Venezia 15, 35131 Padova, Italy INAF–Osservatorio Astronomico di Padova, Vicolo dell’ [email protected] Osservatorio 5, 35122 Padova, Italy e-mail: 16 2 CNR–IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la 17 Astronomía s/n, 18008 Granada, Spain Department of Industrial Engineering – University of Padova, via 3 Department of Physics and Astronomy “G. Galilei”, University of Venezia 1, 35131 Padova, Italy 18 UNITN, Universitá di Trento, via Mesiano, 77, 38100 Trento, Italy Padova, Vicolo dell’ Osservatorio 3, 35122 Padova, Italy 19 4 Max-Planck-Institut für Sonnensystemforschung, Justus-von- Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France Liebig-Weg 3, 37077 Göttingen, Germany 20 5 Laboratoire de Astrophysique de Marseille UMR 7326, CNRS INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy & Aix-Marseille Université, Cedex 13, 13388 Marseille, 21 France Department of Geosciences, University of Padova, via Gradenigo 6, 6 35131 Padova, Italy Centro de Astrobiología, CSIC-INTA, Torrejón de Ardoz, 28850 22 Madrid, Spain Aix-Marseille Université, CNRS, Laboratoire de Astrophysique de 7 Marseille, UMR 7326, 13388 Marseille, France International Space Science Institute, Hallerstrasse 6, 3012 Bern, 23 Switzerland Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 8 Berlin, Germany Research and Scientiﬁc Support Department, European Space 24 Agency, 2201 Noordwijk, The Netherlands Institute of Astronomy, National Central University, 32054 9 Chung-Li, Taiwan Department of Physics and Astronomy, Uppsala University, 75120 25 Uppsala, Sweden Space Science Institute, Macau University of Science and 10 Technology, Macau, PR China PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Polond 26 11 ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain Institute for Geophysics and Extraterrestrial Physics, TU 27 Braunschweig, 38106 Braunschweig, Germany Institut für Datentechnik und Kommunikationsnetze, 12 Hans-Sommer-Str. 66, 38106 Braunschweig, Germany Department for Astronomy, University of Maryland, College Park, 28 MD 20742-2421, USA Department of Information Engineering, University of Padova, via 13 Gradenigo 6/B, 35131 Padova, Italy LESIA, Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. 29 Paris-Diderot, 5 place J. Janssen, 92195 Meudon Pricipal Cedex, Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 France Bern, Switzerland

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